The present invention relates to a timing extraction circuit and a digital communication system, and more particularly, to such a timing extraction circuit and a communication system suitable for use in performing high-speed transmission of above several megabits per second between an exchanger and a terminal through a telephone line as a transmission medium.
In a digital communication system, received data is acquired by discriminating a received waveform in synchronism with a certain reference clock pulse train. For this purpose, a technique of generating the reference clock pulses from a received waveform, that is, a method of extracting a timing signal, plays an important role. In a high speed digital transmission system for transmitting data at a speed above several megabits per second through a telephone line, since signal losses are increased due to the telephone line, and cross-talk noises are also increased, it is desirable to employ a multilevel transmission code having a narrow frequency band. As a transmission code satisfying this condition, a method of employing an LC tank circuit has conventionally been known. In order to extract the necessary clock pulses with, for example, the partial response class-4 (hereinbelow simply referred to as PR4 code), the received signal is raised to the fourth power, and the biquadrate is applied to the LC tank circuit.
On the other hand, regarding low speed digital transmission, there has been also known a method of converting a received waveform to a digital signal by an A/D converter, which are subjected to correlative operations so a to extract the timings.
Also, as a method applicable to a high speed transmission realized by relatively simple hardware, a zero-cross detecting method has been known. Detail of the zero-cross method is disclosed, for example, in IEEE, NTC 1980 65.4, wherein a timings of zero-cross points are detected by a discriminator having a threshold value set at 0 volt, and these signals are inputted to a phase locked loop (PLL) to extract the timing clock pulses. This method is extensively applied to two-level code signal data transmission.
There is also known a method of using a PLL after full-wave rectification it for applying to the three-level AMI code. Such a method is disclosed, for example, in Communication systems CS81-187, Institute of Electronics, Information and Communication Engineers in Japan.